The present invention relates to a novel anti-sunburn skin-care preparation or, more particularly, to an anti-sunburn skin-care preparation formulated with a specific organic compound having strong absorption of ultraviolet light, hereinafter referred to as UV light, but never used as an ingredient of skin-care preparations, which compound is very effective lastingly in the sunburn-preventing effect without the problems of irritation to or sensitization of human skin. The invention also relates to a method for the protection of human skin from sunburn when the skin is exposed to UV light.
Needless to say, the wavelength region of the sun light covers the ultraviolet range which can be divided into three sub ranges including the long-wavelength UV light having a wavelength in the range from about 320 to about 400 nm, referred to as the UV-A light hereinbelow, medium-wavelength UV light having a wavelength in the range from about 280 to about 320 nm, referred to as the UV-B light hereinbelow, and short wavelength UV light having a wavelength shorter than about 280 nm, referred to as the UV C light hereinbelow.
It is known that the UV-B light, among the three types of UV light mentioned above, is responsible for acute inflammation or erythema and burn on the human skin exposed to the light and the UV A light directly causes suntan in the human skin by the oxidation of the reduced melanin in the cuticle. On the other hand, the UV-C light, which is absorbed almost completely in the atmospheric air so that the intensity of the UV-C light arriving at the earth's surface is very low, is believed to be even more harmful against human skin than the UV-B light.
In view of the above described characteristic influences of the UV lights, most of conventional skin-care preparations for sunburn prevention are formulated with an UV absorber capable of absorbing mainly the UV-8 light having the strongest activity to cause skin inflammation, such as benzophenone compounds and the like, preventing the UV-B light from reaching the skin. It is also conventional that a skin-care preparation is compounded with a fine inorganic powder either alone or in combination with the above mentioned UV absorber with an object to shield the UV lights by scattering before the light reaches the human skin.
Anti-sunburn skin-care preparations compounded with an inorganic powder as an UV scattering agent, however, are not always quite satisfactory because of the problems of poor or uneven finishing of make-up and collapsing of the make-up in part to cause inadvertent sunburn at the spots.
On the other hand, results of the dermatological studies obtained in recent years in relation to the influences of UV lights indicate that the UV-A light also has various adverse effects on the human skin exposed thereto such as the phototoxic reactions induced by the exposure to the light, acceleration of the peroxidation of squalene contained in the skin and promoted aging of the human skin. It is also indicated that the UV-A light penetrates the human skin more deeply than the Uv-B light.
Accordingly, it is desired to formulate an anti-sunburn skin care preparation with an UV absorber having excellent absorptivity for the UV-A light. Several compounds have been proposed as a UV A light absorber including 4-(1,1-dimethylethyl)-4'-methoxy dibenzoyl methane disclosed in Japanese Patent Kokai 55-66535, specific benzophenone derivatives disclosed in Japanese Patent Kokai 62-138422 and the like.
The above described UV absorbers conventionally formulated in anti-sunburn skin-care preparations in general have a problem relative to the safety against human body and the sustainability of the anti-sunburn effect because these compounds are soluble in water, organic solvents, oleaginous materials, sebaceous matters and the like to be absorbed percutaneously or to be diffused over the skin surface.